The role of proteins of the transforming growth factor-β superfamily in the intraovarian regulation of follicular development

Ovarian follicular development occurs in a hierarchical manner with each follicle having a unique biochemical composition at any moment in time. It has long been understood that a precise coordination between the growth and maturation of the oocyte and adjacent follicular cells (i.e. somatic cells) is essential in order to produce an oocyte that is fully competent to undergo fertilization and embryo development. In addition to the critical endocrine signalling pathways between the hypothalamus, pituitary and ovary, it is now evident that the oocyte itself is important in influencing the microenvironment of the developing follicle by regulating, via paracrine and autocrine mechanisms, its own maturation as well as somatic cell proliferation, differentiation and ovulation rate. Several of the key oocyte-derived regulating factors are members of the transforming growth factor-beta (TGF-beta) superfamily and to date the best understood are growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15) and BMP6. Significant species differences appear to exist in the relative importance of these growth factors and much remains to be elucidated about their roles in the human ovary. More information on the roles of these factors during ovarian follicular development is likely to advance new therapeutic applications for management of fertility as well as our understanding of how better to assess oocyte quality.